1. Field of the Invention
The present invention relates to an improvement of a varifocal lens used in a camera, etc.
2. Description of the Related Art
In the field of a camera, various kinds of varifocal lenses are used to photograph photographed objects located at long and short distances at desirable sizes and compositions.
In such varifocal lenses, there is a so-called two-focal lens having more than two groups of movable lenses. The two-focal lens is constructed such that one lens group is inserted and separated from a photographing optical path so as to provide first and second focal lengths. For example, the first focal length is set to a short focal distance for performing a standard or wide angle photographing operation. Further, for example, the second focal length is set to a long focal length for performing a telescopic photographing operation.
In a stepwise varifocal lens, a photographing optical system is constructed by e.g., a main lens system and a sublens system. The main lens system is disposed such that this main lens system is displaced along a photographing optical axis. The sublens system is disposed such that this sublens system can be inserted into a predetermined position on the photographing optical axis and can be moved from this predetermined position to an escaping position outside the photographing optical axis. For example, when the photographing operation is performed at the short focal length, only the main lens system is positioned on the photographing optical axis to provide the first focal length. In contrast to this, when the photographing operation is performed at the long focal length, the sublens system is inserted into the predetermined position on the photographing optical axis while the main lens system is displaced along the photographing optical axis, thereby providing the second focal length.
In such a varifocal lens, it is necessary to set or displace the movable lens group to starting positions for providing the respective focal lengths so as to provide the respective focal lengths or change (or switch) one of the focal lengths to the other. Further, it is necessary to displace the movable lens group so as to perform a focusing operation at each of the focal lengths.
There are various kinds of direct and peripheral known techniques relating to the varifocal lens. However, such general techniques have the following problems when the accuracy in displacement of a lens system at the time of the change in focal length (at a zooming time) and the focusing time is held at a high level, and operability is improved and a time for operating the varifocal lens is shortened and the structure of the varifocal lens is made compact.
A first problem in the above known techniques is an irrational problem in human engineering caused in the focusing operation.
Namely, in Japanese Patent Application Laying Open (KOKAI) No. 60-80812, the starting positions of a main lens system at the short and long focal lengths are set to infinite photographing positions (.infin. photographing positions) at the respective focal lengths. Accordingly, when the focusing operation is performed, the main lens system must be generally started from the infinite photographing position even when the photographing operation at the short focal length on the wide angle side is performed to photograph a photographed object located at a short distance. This start of the main lens system from the infinite photographing position causes an irrational problem in human engineering.
This is probably because a cam driving mechanism is used as a switching means for changing or switching the focal lengths in the general varifocal lens. Further, this is probably because the starting positions of the focusing operations at the short and long focal lengths are set to the infinite photographing positions so as to provide simplified structure and system in the above general techniques and such a setting means is used as it is in the case of a lead screw driving system.
However, in the photographing operation at the short focal length at which the photographed object located at the short distance is often photographed, it is natural in view of human engineering to design the varifocal lens such that the main lens system is set in a position on a proximate distance side in advance and the focusing operation with respect to the photographed object at the short distance is performed from the beginning in the photographing operation.
Accordingly, no user can naturally use the varifocal lens and the above irrational problem in human engineering is caused in accordance with the general techniques.
In a second problem, when the starting positions of the main lens system at the short and long focal lengths are set to the respective infinite photographing positions or positions at most proximate distances, an operating time required to change the focal lengths is increased for reasons of the structure of the varifocal lens so that it is troublesome for a user to operate the varifocal lens.
In this case, a region for displacing the lens system in the focusing operation at one of the focal lengths exists between the infinite photographing positions at the short and long focal lengths. Accordingly, the operating time for changing the focal lengths is increased by a moving amount of the lens system for passing through this displacing region. For example, it takes about one second as a total to change the focal lengths.
When the operating time required to change the focal lengths is increased, for example, a shutter chance is lost and it is very inconvenient for a user.
In a third problem, the accuracy in movement of the main lens, etc. is greatly reduced when a means for detecting the position of a movable lens group shown in Japanese Patent Application Laying Open (KOKAI) No. 60-80812 is used to control the displacement of the main lens, etc.
The third problem is an important problem when the means for detecting the position of a movable lens group is used in a control operation for moving the main lens, etc. Further, this third problem is commonly caused in the cases of a varifocal lens having one movable lens group and a varifocal lens of a zooming type.
A fourth problem relates to the relation between a rotational speed of a motor and the maintenance of accuracy in movement of the movable lens group in the case of a driving system using this motor.
To simplify the structure of the varifocal lens of this kind as much as possible, it is advantageous to construct the varifocal lens in view of the number of constructional parts and a space for arranging the varifocal lens such that the switching operation of the focal lengths and the focusing operation at each of the focal lengths are performed by using one motor and one driving means for moving the movable lens group. In this case, this driving means is constructed by e.g., a driving means of a lead screw type.
However, in the case of the driving system using a motor, it is necessary to dispose a gear series means such as a series of reduction gears between the motor and the driving means for moving the movable lens group. Further, a gear ratio is limited by the accuracy in final stopping position of the used motor so that it takes much time to switch the focal lengths.
If the other conditions except for the above gear ratio are neglected, the accuracy in drawing-out operation of the main lens, etc. can be improved as the reduction gear ratio is increased.
However, a drawing-out time of the main lens, etc. is increased in proportion to a magnitude of the reduction gear ratio.
The increase in drawing-out time of the main lens, etc. does not cause any serious problem when the drawing-out amount of the main lens, etc. is small as in the focusing operation. However, the increase in drawing-out amount of the main lens, etc. causes a problem when the main lens, etc. are moved by a long distance as in the switching operation of the focal lengths. This is because it is preferable to reduce the switching time of the focal lengths as much as possible so as not to lost a shutter chance in the varifocal lens of this kind.
To solve this problem, it is necessary to use a motor having a very high rotational speed. However, much time and cost are required to develop such a motor of a high rotational speed type.
In Japanese Utility Model Application Laying Open (KOKAI) No. 55-91518, it is considered to constitute the varifocal lens such that two motors are basically prepared in advance and one of them is used to switch the focal lengths and the other is used to perform the focusing operation at each of the focal lengths. However, in such a case, it is necessary to arrange the two motors, and driving circuits and mechanisms, etc. attached to the respective motors so that it is not advantageous in consideration of structure and space.
In Japanese Utility Model Application Laying Open (KOKAI) Nos. 57-63334, 58-77306 and 55-91518, it is possible to construct the varifocal lens such that a circuit for supplying an electric current to a motor is electrically controlled if only a change in rotational speed of the motor is considered. However, in such a case, it is usually necessary to dispose a complicated and dedicated circuit such as a calculating circuit for control so that it is not advantageous in structure.
The above problems are commonly caused in the cases of the varifocal lens having one movable lens group and the varifocal lens of a zooming type. Therefore, it is desirable to solve the above-mentioned problems.
As mentioned above, there are various kinds of problems for providing a varifocal lens having a higher quality in the above general techniques. Accordingly, it is very desirable to take measures for inclusively solving the above-mentioned problems.